restriction on proxy_store based on the size of the given file

Question

Nginx

There is the following logic:
the content is served from the proxy server,
if there is no file, the proxy server accesses http_backend and takes the file from there to its storage, gradually giving it away.

In the config, it looks like this:

    location / {
            aio on;
            directio 1m;
            root /mnt/data;
            try_files $uri @download_it;
    }

    location @download_it
    {
            aio on;
            directio 1m;
            root /mnt/data;
            proxy_store  on;
            proxy_store_access     user:rw group:rw all:rw;
            proxy_pass http://http_backend;
    }

The problem is that during peak hours, when there is an active return, if a large file appears (for example, 30 GB), then writing it to disk, small files lose the return a little, but this is not critical.

my question is whether it is possible to somehow read the response header http_backend and if the file is more than 4 gigabytes goes into the
proxy_store off mode; well, or even do a redirect to the server. on which the file exists.

The question is reduced to that how to make check on the heading received from an Upstream?